1. Field of the Invention
The present invention relates to copy operations performed upon data that resides in direct access storage device (xe2x80x9cDASDxe2x80x9d). More particularly, the invention concerns a method of adapting instant virtual copy methods to data objects that span storage boundaries such as logical and/or physical DASD subsystems.
2. Description of the Related Art
A number of DASD subsystems are capable of performing xe2x80x9cinstant virtual copyxe2x80x9d operations, also referred to as xe2x80x9cfast replicate functions.xe2x80x9d Instant virtual copy operations operate by modifying metadata such as relationship tables or pointers to treat a source data object as both the original and copy. In response to a host""s copy request, the storage subsystem reports creation of the copy without having made any physical copy of the data. Only a xe2x80x9cvirtualxe2x80x9d copy has been created, and the absence of any physical copy is completely unknown to the host.
Later, when the storage system receives updates to the original or copy, the updates are stored separately and cross-referenced to the updated data object only. At this point, the original and copy data objects begin to diverge. The initial benefit is that the virtual copy occurs almost instantaneously, and at any rate, much faster than a normal, physical copy operation. This frees the host and storage subsystem to perform other tasks. The host or storage subsystem may even proceed to create an actual, physical copy of the original data object during background processing, or at another time.
With such benefits, instant virtual copy has been an important development in modern DASD subsystems, and a number of different embodiments have surfaced. As one example, International Business Machines Corporation (xe2x80x9cIBMxe2x80x9d) has developed the xe2x80x9cflash copyxe2x80x9d technique, as described in U.S. application Ser. No. 09/347,344, filed on Jul. 2, 1999 and entitled xe2x80x9cMethod, System, and Program for Maintaining Electronic Data as of a Point-In-Time.xe2x80x9d Another example is the xe2x80x9csnapshotxe2x80x9d technique disclosed in U.S. Pat. No. 5,410,667 entitled xe2x80x9cData Record Copy System for a Disk Drive Array Data Storage Subsystem,xe2x80x9d which issued on Apr. 25, 1995. The foregoing references are hereby incorporated herein by reference.
Although these techniques are useful in many applications, there are still certain limitations. Namely, known instant virtual copy techniques do not apply to data objects that span storage boundaries such as logical or physical DASD subsystems. For example, if a data object occupies storage that has been allocated to different log structured arrays, the snapshot technique cannot be used to create an instant virtual copy of the data object.
This problem is especially acute in today""s data storage subsystems, where data objects frequently span storage boundaries for a number of different reasons. For one, multi volume datasets often span storage boundaries because of their large size. As another example, data objects sometimes end up spanning storage boundaries because of xe2x80x9csystem managed storage,xe2x80x9d where the storage system rather than the host selects the optimum storage strategy to meet requirements of access time, data redundancy, etc. As still another reason, data objects necessarily span storage boundaries to implement xe2x80x9cstripingxe2x80x9d or other data distribution schemes designed to minimize the effect of a catastrophic failure of one storage component.
Therefore, modern data storage subsystems frequently present situations where data objects span storage boundaries. However, as described above, known instant virtual copy techniques are not available in these situations. Without the instant virtual copy technique, copy requests are performed by the host, which reads source data from storage into host memory and then rewrites the data to target storage. Undesirably, this process demands more time than the instant virtual copy process, and also consumes valuable host processing resources. These demands are likely to be high because the data objects that span storage boundaries are likely to be large, due to the fact that they occupy enough space to span storage boundaries. In these situations, then, known instant virtual copy techniques may not be completely adequate due to certain unsolved problems.
Broadly, the present invention concerns the adaptation of instant virtual copy techniques to data objects that span storage boundaries such as logical and/or physical DASD subsystems. Initially, a data storage system receives a request to copy a data object contained in the system. The storage system includes multiple storage regions separated by storage boundaries. The regions may correspond to logical or physical DASD subsystems, for example. The storage system determines whether the data object resides in more than one of these regions. If so, the storage system divides the data object into multiple subparts, each contained within a single storage region. Then, the storage system separately performs an instant virtual copy operation for each subpart. On the other hand, if the data object already resides in a single one of the regions, the storage system can perform an instant virtual copy operation for the entire data object as a whole.
Accordingly, one embodiment of the invention constitutes a method to make instant virtual copies of data objects that span storage boundaries. In another embodiment, the invention may be implemented to provide an apparatus, such as a data storage system, configured to make instant virtual copies of data objects despite their spanning of storage boundaries. In still another embodiment, the invention may be implemented to provide a signal-bearing medium tangibly embodying a program of machine-readable instructions executable by a digital data processing apparatus to perform copy operations as discussed above. Another embodiment concerns logic circuitry having multiple interconnected electrically conductive elements configured to perform copy operations as discussed above.
The invention affords its users with a number of distinct advantages. Chiefly, the invention enables a storage system to copy a data object virtually immediately, regardless of how that data object is distributed in the storage system. Accordingly, the invention conserves host processing capability, since copy operations are completed quickly. Thus, processing resources are more available for other tasks, such as satisfying read/write requests, mirroring data to remote storage sites, running application programs, etc. The invention also provides a number of other advantages and benefits, which should be apparent from the following description of the invention.